Two principal capacitive sensing and measurement technologies are currently employed in most capacitive touch sensing devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC.™ Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance may be measured through the detection of the amount of charge accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the grounded object is a finger. The human body is essentially a capacitor to a surface where the electrical field vanishes, and typically has a capacitance of around 100 pF.
Electrodes in self-capacitance touchscreens and/or touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual disturbances induced by the presence of a finger, for example, can be determined.
The second primary capacitive sensing and measurement technology employed in capacitive touch sensing devices is that of mutual capacitance, where measurements are typically performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999. In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
In some mutual capacitance measurement systems, an array of sense electrodes is disposed on a first side of a substrate and an array of drive electrodes is disposed on a second side of the substrate that opposes the first side, a column or row of electrodes in the drive electrode array is driven to a particular voltage, the mutual capacitance to a single row (or column) of the sense electrode array is measured, and the capacitance at a single row-column intersection is determined. By scanning all the rows and columns a map of capacitance measurements may be created for all the nodes in the grid. When a user's finger or other electrically conductive object approaches a given grid point, some of the electric field lines emanating from or near the grid point are deflected, thereby decreasing the mutual capacitance of the two electrodes at the grid point. Because each measurement probes only a single grid intersection point, no measurement ambiguities arise with multiple touches as in the case of some self-capacitance systems. Moreover, it is possible to measure a grid of n×n intersections with only 2n pins on an IC.
Several problems are know to exist in respect of the operation of prior art mutual capacitance touchscreens, however, including, but not limited to, distinguishing real finger touches from hovering finger touches, an inability to predict with any certainty where a user is likely to place his finger on a touchscreen next, noise signals interfering with touch signals, significant variability among different users with respect to their touch habits and motions, undesired changes in operational characteristics arising from changes in the ambient environment or changing finger sizes or user habits, and high power consumption that may be induced by false wakeups.
Improved methods of operating a mutual capacitance sensing system are required to permit more accurate and adaptable touch sensing, as well as reduced power consumption.